use constant HAS_HTML_TIDY => do {
    local $@;
    eval { require HTML::Tidy; 1 } or do {
        unless ( eval { require XML::Clean; 1 } ) {
            die( "Please install HTML::Tidy (preferred) or XML::Clean first" );
        }
        0; # Has XML::Clean but no HTML::Tidy
    };
};
...
sub parse_file {
    ...
    if (HAS_HTML_TIDY()) {
}